A slow-wave structure, with phase velocity substantially slower than the speed of light, typically finds application in Traveling-Wave Tube (TWT). The TWT is an amplifier of microwave signals and it provides the largest bandwidth among all high power vacuum electronic devices. Two primary components of a TWT are an electron beam (e-beam) and a travelling electromagnetic (EM) wave. The EM wave is guided by a slow-wave structure. The slow-wave structure slows down the EM wave, ensuring ‘velocity synchronism’ between the electrons in the e-beam and the EM wave.
The most common slow-wave structure is the circular helix because of its un-matched capability for strong electron-wave interaction over large bandwidths. However, the circular helix is not a planar structure and it is not amenable to fabrication using printed-circuit or micro-fabrication techniques. Printed-circuit techniques are important for miniaturization as well as low-cost mass-production. Miniaturized TWTs can have widespread applications in communications, radar, spectroscopy etc. Moreover, since device dimensions scale inversely with frequency, at high frequencies the fabrication of the electron gun and slow-wave structure using conventional manufacturing technology becomes very difficult. Therefore micro-fabrication techniques are almost mandatory at high frequencies of operation. Further, an advantage of a planar slow-wave structure is the possibility of use of sheet geometry for electron beam. As compared to the round beam geometry, sheet beam geometry offers advantages of higher beam current capacity, decreased beam voltage and increased bandwidth.